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Abstract

It has been found that there is a relationship between drop velocity and drop volume in drop on demand ink jet printers. The drop volume is a linear function of the drop velocity. Drop size modulation is the key to gray scale printing techniques. Therefore, in order to print a quality gray scale, it is necessary to compensate for the velocity variations, assuming that the path of volume modulation is chosen. This article teaches the applicability of a known circuit technique to the solution of this compensation problem. Of course, the problem could also be solved in software, but the computations may prove burdensome, particularly in a fast printer with a large number of nozzles. The circuit concept proposed here is that embodied in U.S. Patent 4,158,204.

Country

United States

Language

English (United States)

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Velocity Compensation For Drop On Demand Printers

It has been found that there is a relationship between
drop velocity and drop volume in drop on demand ink jet printers.
The drop volume is a linear function of the drop velocity. Drop size
modulation is the key to gray scale printing techniques. Therefore,
in order to print a quality gray scale, it is necessary to compensate
for the velocity variations, assuming that the path of volume
modulation is chosen. This article teaches the applicability of a
known circuit technique to the solution of this compensation problem.
Of course, the problem could also be solved in software, but the
computations may prove burdensome, particularly in a fast printer
with a large number of nozzles. The circuit concept proposed here is
that embodied in U.S. Patent 4,158,204.

The essence of the problem is to find a simple circuit which has for input the
drop volume and the desired position on the page, and which has as output a
timing signal which triggers the emission of a droplet. More fundamentally, the
input would be the desired gray scale value which is also the input to the circuitry
which shapes the driving waveform so as to produce the desired drop volume.

The starting point for the circuit is a clock which emits a
pulse for every "zeroorder" pel (picture element) position on the
raster line for all nozzles (assuming an array). Thus, if there are
100 pels/cm, and the nozzle velocity is 100 cm/sec, this "zero clock"
(ZC) frequency is 10 kHz. A subsidiary or scan clock (SC) is also
needed which divides the base period into possible drop firing times.

For definiteness, 10 possible gray levels (10 possible drop volumes)
are assumed with the drop volumes in the ratios 1:2:3....:10. Other
volumes and ratios, of course, are possible. In this simple case,
there are ten different velocities and, therefore, ten different
flight times. Hence, 10 subscan periods, or a frequency of 100 kHz
is needed. (For simplicity, it is assumed that the nozzle
translation velocity has been chosen to be the maximum that would
allow all possible drop volumes to be printed. Obviously, the
subscan frequency can be adjusted to whatever value is needed to
introduce the required delays between different sized dro

In the referenced patent, it is shown that velocity variations can be
compensated for by use of a delay register (DR), which is, in essence,
programmable based on information about nozzle to nozzle velocity variations.
The input to the nozzle driver is stored in an information or write register (WR),
and at each subscan period the delay register is examined to determine whether
to output a bit in the WR to the nozzle driver. Thus, a nozzle is energized with a
simple AND between the...